Monochromators having concave gratings currently available on the market and operating by simple rotation of the gratings find their efficiency rapidly decreased in the ultraviolet band (.lambda. &lt; 800A) due to the fact that the angles of incidence at which these devices operated are relatively small, for example, less than 35.degree..
If a monochromator having simple rotation of the grating is used for large deviations, the losses through astigmatism become very substantial with a correspondent loss in luminosity. One can attempt to correct a part of the astigmatism by utilizing a holographic grating. However, for relatively small angles of incidence, and for short wave lengths, this astigmatism is too substantial to be able to be corrected solely by the holographic grating.